minor #606 Updated Symfony to 3.3.4 (javiereguiluz)

This PR was merged into the master branch.

Discussion
----------

Updated Symfony to 3.3.4

Changelog:

  - Updating erusev/parsedown (1.6.2 => 1.6.3)
  - Updating ezyang/htmlpurifier (v4.9.2 => v4.9.3)
  - Updating symfony/polyfill-apcu (v1.3.0 => v1.4.0)
  - Updating symfony/polyfill-mbstring (v1.3.0 => v1.4.0)
  - Updating twig/twig (v1.33.2 => v1.34.4)
  - Updating twig/extensions (v1.5.0 => v1.5.1)
  - Updating symfony/polyfill-util (v1.3.0 => v1.4.0)
  - Updating symfony/polyfill-php70 (v1.3.0 => v1.4.0)
  - Updating symfony/polyfill-php56 (v1.3.0 => v1.4.0)
  - **Updating symfony/symfony (v3.3.0 => v3.3.4)**
  - Updating symfony/polyfill-intl-icu (v1.3.0 => v1.4.0)
  - Updating symfony/polyfill-php54 (v1.3.0 => v1.4.0)
  - Updating phpunit/phpunit (4.8.35 => 4.8.36)
  - Updating symfony/phpunit-bridge (v3.3.0 => v3.3.4)
  - Updating monolog/monolog (1.22.1 => 1.23.0)
  - Updating gecko-packages/gecko-php-unit (v2.0 => v2.1)
  - Updating symfony/polyfill-php55 (v1.3.0 => v1.4.0)
  - Installing symfony/polyfill-php72 (v1.4.0)
  - Removing symfony/polyfill-xml (v1.3.0)

Commits
-------

e5b430d Updated Symfony to 3.3.4

Author: javiereguiluz